The nitration of aromatic compounds, especially benzene, to produce mononitrated aromatic compounds, which process is hereinafter sometimes discussed with reference to the nitration of benzene to produce mononitrobenzene, hereinafter referred to as nitrobenzene, is widely practiced commercially, particularly for the manufacture of aniline. Conventional commercial processes for nitrobenzene ordinarily utilize either a batchwise or continuous addition of a mixture of sulfuric acid and nitric acid, commonly referred to as mixed acid, to the aromatic charge material.
Representative processes for the nitration of aromatic compounds are disclosed by Castner in U.S. Pat. No. 2,256,999; and Alexanderson et al. in U.S. Pat. Nos. 4,021,498 and 4,091,042, respectively.
In these processes, and others, after the nitration reaction is completed, the spent acid, i.e., the mixed acid essentially completely depleted of nitric acid, is separated from the nitroaromatic compound layer. The crude nitroaromatic compound layer is then washed with (1) water, or (2) water containing ammonia or an alkali metal base, to remove water-soluble acidic impurities and oxidation by-products such as e.g., dinitrophenols and picric acid. The resulting wash waters, saturated with nitroaromatic compound, are normally either sewered directly, or steam stripped, or adsorbed on carbon to recover residual nitroaromatic compound before being discharged.
The direct sewering of the wash waters results in a 0.1% to 0.5% loss of yield of nitroaromatic compound and furthermore, since the effluent is not treated, it is toxic to aquatic life. The treatment of this effluent in a biological effluent treatment plant, however, is difficult because nitrated aromatic compounds are also toxic to the bacteria used therein. In view of the harmful nature of e.g., nitrobenzene, and the difficulty in treating effluents containing e.g., nitrobenzene, in modern commercial practice the nitrobenzene is usually stripped from the wash liquors with steam. This method reduces the nitrobenzene in the effluent wash water to a level acceptable for subsequent biological effluent treatment, but it is at the expense of about 40 pounds of steam per pound of nitrobenzene recovered. Thus, the stripping operation is costly in equipment and energy.